52 Project 2012: Foundational Electronic Components #5 – Transistors

What is a transistor?

A transistor is a semiconductor-based device that allows one electrical signal to logically control the behavior of another signal.

How does a transistor work?

Fundamentally, a transistor is just two NP-semiconductor diodes with the same type of semiconductor layers mashed together. If the two “N” layers are back-to-back, then transistor is called PNP, while if the two “P” layers are back-to-back, the transistor is called NPN. The center layer acts as a logical switch (requiring very little power to use) which toggles the flow of current between the other two layers. One of the outside layers is called the collector, the middle layer is called the base, and the other outside layer is called the emitter.

A transistor is one of the most influential electronic components in existence today. It is the basic building block of computers, allowing one electrical signal to control the behavior of another. Transistors provide logical behavior to be designed and implemented into machines. Multiple transistors can be combined to create what are called “logic gates” (allowing AND, OR, XOR, and other operations), and these can be further combined into more complex systems. As technology allowed transistors to be manufactured smaller and smaller, more could be combined into a single device. Some extremely powerful CPUs contain over six billion transistors all in one module. This is phenomenal.

In addition to logical switching, transistors can also amplify electrical signals. This amplification ability is what allowed widespread inexpensive transistor radios, and the replacement of old vacuum-tube amplifiers with cheaper and more reliable “solid-state” circuits, based on transistors. A very small current into the base can result in a much larger current between the collector and emitter.

Transistors, unlike first four components we’ve explored, have three terminals. One terminal is connected to the collector, one to the base, and one to the emitter.

Transistor ratings

Aside from the basic “PNP” or “NPN” classification, transistors have quite a few different ratings. Because the physical materials are based on the same technology that diodes use, two critical ratings are power dissipation and reverse voltage limits. Transistors also have maximum collector current rating.

Since transistors are in many cases intended to amplify, paying special attention to the maximum power dissipation is critical, since amplification circuits often require heatsinks and still get very hot.